The invention relates to multistage ejectors. In particular, the invention relates to multistage ejectors of the type comprising a nozzle arrangement which has at least three nozzles which are arranged in series in the direction of a longitudinal axis.
A multistage ejector of the type referred to in the introduction is used for creating a vacuum or negative pressure. Objects can be lifted, for example, with the negative pressure which is created. For this purpose, the ejector is connected to a corresponding lifting device, wherein the negative pressure which is created by the ejector acts upon a suction gripper by means of which the object is lifted.
In general, the functioning principle of an ejector is based on Bernoulli's law, according to which the static pressure in a flow drops as flow velocity increases. The ejector is operated with a fluid, especially compressed air, as the driving medium which flows at high velocity through the nozzle arrangement. A static negative pressure is thereby created at the fluid gaps between the individual nozzles. This reduced static pressure is used to draw in medium from a space surrounding the ejector—in the case of air being used for lifting—via one or more suction ports in the sleeve of the ejector which communicate with the fluid gaps between the individual nozzles.
When in use, the ejector is installed in an ejector housing, wherein the ejector housing has a bore, the inside diameter of which is matched to the outside diameter of the sleeve so that the sleeve is accommodated in the bore with no clearance. The ejector housing has ports for the feed of fluid as driving medium and ports for drawing in suction medium.
A multistage ejector of the prior art is distributed by the SMC Company under the multistage ejector type designation “ZL 212”. This known ejector has in all four nozzles, these being a driver nozzle and three receiver nozzles, as seen in the flow direction of the fluid, wherein all the nozzles are arranged in series in the direction of a longitudinal axis. The three receiver nozzles of this known ejector are interconnected monolithically, i.e. in one piece to form a nozzle string. The nozzle string is accommodated in a sleeve, wherein the nozzle string and the sleeve are detachably fastened to each other.
In the case of the known ejector, the nozzle string and the sleeve are fastened via a frictional engagement which is brought about by means of seals which are arranged on the nozzle string on the outer side and bear against the inner wall of the sleeve with sealing effect.
Now it is necessary from time to time to remove the ejector from the ejector housing for maintenance purposes, wherein to this end the ejector has to be withdrawn from the bore of the ejector housing. Since in the case of the known ejector the nozzle string is only fastened on the sleeve by frictional engagement, it can happen when the ejector is being withdrawn from the ejector housing that only the nozzle string is withdrawn from the sleeve while the sleeve remains fitted in the ejector housing and can then only be removed from the ejector housing by increased manipulating effort. It would admittedly be possible to increase the frictional engagement between the nozzle string and the sleeve so that when the ejector is being withdrawn from the housing the nozzle string is not separated from the sleeve prematurely, but this leads to a more complicated handling of the ejector during its dismantling into nozzle string and sleeve since the separating of the nozzle string from the sleeve is then associated with increased expenditure of effort. The handling- and maintenance friendliness of the known ejector is therefore disadvantageously reduced.
A further multistage ejector is known from EP 1 064 464 B3. In the case of this known ejector, the individual nozzles are provided with connecting means at the ends in each case so that the individual nozzles can be directly interconnected to form a nozzle body. This known ejector does not have a sleeve.
The disadvantage of this known ejector is that the ejector has to be assembled from a large number of individual parts since all the nozzles exist as individual nozzles. Furthermore, as in the case of the previously referred-to known ejector, there is the disadvantage that when the ejector is being withdrawn from the ejector housing the nozzles are separated prematurely from each other and individual nozzles then remain fitted in the housing. Therefore, the handling and the maintenance friendliness are not satisfactory in the case of this known ejector either.
A multistage ejector, in which the nozzle arrangement has a plurality of individual nozzles which are initially retained individually within an inner frame, is known from EP 1 969 234 B1. The inner frame is installed in an outer sleeve and by means of a locking structure the inner frame and the outer frame are fastened to each other in a rotationally secured manner. The disadvantage of this known ejector is in its multipart construction because it has individual nozzles, an inner frame and an outer sleeve so that the construction of this known ejector is very costly. Associated with this, the assembly of this known ejector is also time consuming.